The subject matter disclosed herein generally relates to phase identification and in particular to phase identification at distribution transformers.
Electric power distribution networks are used by electric utility companies to deliver electricity from generating stations to customers. The distribution voltages vary across utility companies within a country. In a typical power distribution network, three-phase power at a high voltage is delivered to multiple transmission substations. At these transmission substations, high-voltage power is stepped down to an intermediate three-phase voltage. The intermediate-voltage three-phase power from each substation is then delivered to multiple distribution substations. At the distribution substations, the intermediate-voltage is stepped down to a lower distribution voltage and separated into three single-phase feeder lines. Each of these feeder lines branches into multiple circuits to power multiple distribution transformers that step the voltage down to a final single-phase voltage for delivery to the commercial and residential customers.
It is desirable to have the feeder circuits with balanced loads such that the current loads on each single-phase output of a three-phase transformer are equal. However, over time, as customers are added and removed, the loads on each single-phase output may change and become unbalanced. To re-balance the loads, some of the branch circuits are typically moved from a more heavily loaded phase to a more lightly loaded phase. If the phase of each line in the feeder circuit is not accurately known, a line may be erroneously removed from a more lightly loaded phase and placed on a more heavily loaded phase. Such errors result in the procedure having to be repeated, which causes a second disruption in service to all customers on the branch being re-phased. Adding a greater load to the more heavily loaded phase may cause imbalance loading on substation that may result in a power outage for all customers on the overloaded phase.
To identify the phase of a particular feeder branch using conventional manual techniques, utility company personnel must physically trace a line run back through various distribution cabinets until the line reaches a point in the distribution network at which the phase is definitely known. This can be a time consuming, labor-intensive process.
Various devices and methods have been implemented to attempt to identify phase, such as by use of modems and telephone lines to establish a communication link. A signal associated with the phase at a point in the network where the phase of the line is known (the reference line) is transmitted over the communication link to a point in the network where the phase of the line is not known (the line under test). Difficulties arise when delays in the communication link affect the accuracy of the phase measurement.
One method to reduce communication delays implements a communication link via radio transmissions. Another method is to compare phases between electric power system substations in real time via time base synchronization through Global Positioning System (GPS) data from both the reference line and the line under test to eliminate delay and synchronization problems.
In these approaches, a pre-established real-time communication link is required. That is, a communication link needs be established and active at the time the phase of the line under test is measured. This renders these approaches unusable wherever and whenever the communication link cannot be established. Also, because the phase of the line under test is determined for each measurement, the measuring apparatus must be retrieved after each test. This precludes the ability to make several different tests before accessing the apparatus, e.g., measuring the phases of several different overhead lines in a substation before lowering the “hot stick” to which the apparatus is attached.
Accordingly, there is a need to provide an improved apparatus and method for the identification of line phase of a power line in a three-phase power distribution network.